The present invention relates to a recording disc having spiral or concentric turns of a tracking servo groove and a method for fabricating such discs.
One approach to high density disc recording involves forming a tracking servo groove on the disc surface to allow a recording or reading laser beam to precisely follow a desired path.
A recording disc of the type shown and described in Japanese Patent Laid-open Publication 58-57643 is provided with a spiral pattern of tracking servo groove. The groove is modulated in depth so that lands are created at specified positions of tracks. The edges of the groove and the lands are sensed by diffraction of the laser beam to derive a tracking servo control signal and a position indicating signal. A recording layer is deposited on the groove to eraseably store information signals. However, the depth modulation results in the generation of a significantly high amplitude position signal which interferes with the information signal if the range of depth variations, or modulation index, is greater than a certain value, which is approximately 10%. However, this value of modulation index would require the disc surface to vary in depth within a range of 100 Angstroms which is impractical to manufacture.